The invention relates to power-on reset circuits which establish initial conditions for circuitry immediately after power is turned on.
"Power-on" can occur when the circuitry is installed in a system and a system power switch is subsequently activated or when the circuitry resides on a card which is initially disconnected from the system and then installed in the system after the power switch has been activated. For many types and arrangements of integrated circuits, it is important that the circuitry assume a predetermined "safe" state immediately after power-on and until the circuit begins normal operation. For example, tristate CMOS drivers cannot tolerate large heat dissipation, and this can cause a problem in a computer system where the CMOS drivers face each other in a bidirectional communication line. Overheating could occur if one CMOS driver exhibited a high output state whereby it outputted a high level of drive current and the other CMOS driver simultaneously exhibited a low output state whereby it sunk the high level of drive current supplied by the one CMOS driver. Thus, during the period between power-on and beginning of normal operation, it is important that either both drivers be disabled into the tristate (a high impedance output state), or if enabled that the foregoing condition be avoided. There are many other circuit types and arrangements where the circuit states should be controlled during this period.
In the prior art it was known to sense the power supply voltage during power-on, and when the voltage reached the rated voltage or a predetermined voltage near the rated voltage, then generate a reset pulse to initialize the circuitry to a safe state. Also, some prior art power-on reset circuits included delays to the reset pulse to ensure that all circuitry has attained a stable operating state after the rated voltage or predetermined voltage is reached.
While such voltage sensing power-on reset circuits are effective in many types of circuits, they require analog type circuitry which cannot readily be integrated with some types of digital circuitry used to implement the normal function.
Accordingly, a general object of the present invention is to provide a power-on reset circuit which utilizes digital circuitry.